关于android的坑

　　

坑1：

使用SQLiteOpenHelper的时候如果建立的表中存在不为空的字段，但是用ContentValues()的方式来插入数据的话恰好没有往这个字段里插入数据，那么执行后市没法往数据库里插入数据的。单是写在这里可能觉得这个问题很平常，下面以代码的形式展示出来。

MySqliteDatabase.java

package com.jikexueyuan.hellonotes;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;

public class MySqliteDatabase extends SQLiteOpenHelper {
    public static final String _ID = "_id";
    public static final String CONTENT = "content";
    public static final String TIME = "time";
    public static final String TABLE = "mydiary";
    public static final String TABLE_DRAFT = "draft";
    public static final String DATABASENAME = "wyldb.db";

    public MySqliteDatabase(Context context, String name,
            CursorFactory factory, int version) {
        super(context, name, factory, version);
        // TODO Auto-generated constructor stub

    }

    /**
     * 三个参数的构造器
     * 
     * @param context
     * @param name
     * @param factory
     */
    public MySqliteDatabase(Context context) {
        this(context, DATABASENAME, null, 1);
        System.out.println("MySqliteDatabase.MySqliteDatabase()构造器，新建表："
                + DATABASENAME + "...");
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        System.out.println("MySqliteDatabase.onCreate(),创建表...");
        db.execSQL("create table mydiary(_id integer primary key autoincrement,content text not null,time text not null)");
        db.execSQL("create table draft(_id integer primary key autoincrement,content text not null,time text not null)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub

    }

}

DetailActivity.java

package com.jikexueyuan.hellonotes;

import java.text.SimpleDateFormat;
import java.util.Date;

import android.app.Activity;
import android.content.ContentValues;
import android.content.Intent;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class DetailActivity extends Activity implements OnClickListener {
    EditText et_detail;
    Button btn_edit, btn_delete, btn_detail_save;
    String val;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.detail);
        et_detail = (EditText) findViewById(R.id.et_detail);
        btn_edit = (Button) findViewById(R.id.btn_edit);
        btn_delete = (Button) findViewById(R.id.btn_delete);
        btn_detail_save = (Button) findViewById(R.id.btn_detail_save);
        btn_edit.setOnClickListener(this);
        btn_delete.setOnClickListener(this);
        btn_detail_save.setOnClickListener(this);
        Intent intent = getIntent();
        val = intent.getExtras().getString(FirstActivity.KEY_CONTENT);
        if (!val.equals("") || val == null) {
            // Toast.makeText(this, "获取的值不为空，已经设置，，", 0).show();
            et_detail.setText(val);
            et_detail.setSelection(val.length());// 设置光标到最后
        }

    }

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        MySqliteDatabase db = new MySqliteDatabase(DetailActivity.this);
        SQLiteDatabase sdb = db.getWritableDatabase();
        switch (v.getId()) {
        case R.id.btn_edit:
            et_detail.setEnabled(true);// 设置文本克可编辑
            // 可编辑的时候删除按钮隐藏，保存按钮可见
            btn_delete.setVisibility(View.GONE);
            btn_detail_save.setVisibility(View.VISIBLE);
            System.out.println("===val:" + val + ",====");
            // 同时还要删除
            sdb.delete(MySqliteDatabase.TABLE, MySqliteDatabase.CONTENT + "=?",
                    new String[] { val });// 有报错，暂时不处理
            break;
        case R.id.btn_detail_save:
            String content = et_detail.getText().toString();
            ContentValues cv = new ContentValues();

            SimpleDateFormat format = new SimpleDateFormat();
            Date date = new Date();
            String time = format.format(date);
            cv.put(MySqliteDatabase.CONTENT, content);
            /*
             * 之前没有插入time这个值，但是因为这个表里的time字段不能够为空，导致下main的代码都
             * 没正确执行，即不能够按照预期把文本内容插入到数据库。
             */
            cv.put(MySqliteDatabase.TIME, time);
            System.out.println("-----btn_detail_save：content=  ----");
            sdb.insert(MySqliteDatabase.TABLE, null, cv);
            Intent intent = new Intent(DetailActivity.this, FirstActivity.class);
            startActivity(intent);
            finish();
            break;

        default:
            break;
        }
    }
}

 具体代码：http://pan.baidu.com/s/1sjIJRit

坑2：viewpager与fragment一起用的时候，需要传入fragment作为参数，在自己的Fragment实现类中有一个必须复写的方法，

public View onCreateView(LayoutInflater inflater,
@Nullable ViewGroup container, @Nullable Bundle savedInstanceState){

}

，具体的坑如下：

public class Fragment2 extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater,
            @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        // return inflater.inflate(R.layout.view2, container, true);
        // 这里的第二个参数只能填写null,不能像上面一行注释的那样填写container，
        // 否则会报错：The specified child already has a parent. You must call
        // removeView() on the child's parent first.
        return inflater.inflate(R.layout.view2, null, true);
    }
}

同时也可以参见：http://blog.csdn.net/mcy478643968/article/details/26812225 ，具体源码可见百度网盘里的Wyl.rar。